Polar photocatalyst materials usually exhibit ferroelectric characteristics giving rise to spontaneous polarization behavior which works as a driving force for the separation of photogenerated electrons and holes and mitigates the effect of charge recombination. This study shows that the surface potential changes for a polar phtotocatalyst before and after photoirradiation can be used to predict the photocatalytic activities among different phtotocatalysts. We systematically investigated the correlation among the surface properties, crystal structure, electronic band structure, photocatalytic activity, and stability of four B O and alkaline earth cations containing photocatalysts, M2B5O9Cl (M = Ca, Sr, Ba, Pb). Among the four studied photocatalysts, Ba2B5O9Cl exhibits the greatest changes in the surface potential after photoirradiation and also shows the highest photocatalytic activity for dechlorination of chlorophenols under UV light irradiation. Its photocatalytic activity is about 1.3, 2.8, 4.4, and 15 times those of Ca2B5O9Cl, Sr2B5O9Cl, Pb2B5O9Cl, and P25 samples, respectively. The results support that the photocatalytic activity of the four photocatalysts strongly depends on the spontaneous polarization power. Overall, these findings demonstrate the utility of Kelvin probe force microscopy that can screen for a highly efficient photodegradation materials in the field of photo catalysis.